The distribution of so-called inorganic elements in biological samples is important to determine for numerous reasons. Inorganic elements generally refers to elements other than those that typically form organic material such as C, H, N and O. Usually, the inorganic elements of interest are heavier than oxygen and typically are metallic or semi-metallic elements. The natural distribution of inorganic elements in biological samples reveals important information about biological processes at gene, protein and metabolite levels as reflected by the burgeoning field of metallomics. In addition, in an approach called elemental tagging, a number of so-called elemental tags (which may also be termed markers) can be added artificially to targets in the sample, typically with the help of specific binding agents (for example antibodies, aptamers, metabolic labels, etc.) to focus on specific targets or processes in biological systems. Many different detection techniques can be employed for measuring the abundance of the elements of such tags, such as radioactivity, light (e.g. fluorescence or absorption), which includes X-ray fluorescence (XRF), secondary electron spectrometry (SES), X-ray photoelectron spectroscopy (XPS), electron micro-probe analysis (EMPA), secondary ion mass spectrometry (SIMS), laser plasma ionisation mass spectrometry (LPI MS) and inductively-coupled plasma mass spectrometry (ICP MS), etc.
In the case of fluorescence based assays, the techniques may be fast but suffer from low sensitivity and are limited to one or a few targets per assay in comparison to mass spectrometric techniques such as SIMS or ICP MS.
Mass spectrometry techniques allow a high degree of multiplexed measurement of elements in parallel, for example using multi-collector magnetic sector, time-of-flight, ORBITRAP or Fourier transform ion cyclotron resonance analyzers. However, when spatially resolved analysis is required, for example for imaging of tissues, low abundance of elements poses a challenge to all these analyzers as spectra become dominated by intense matrix peaks from tissues. These matrix peaks could originate from polyatomic species constituting bulk of tissues, with major elements being not only C, H, N, O, but also S, P, alkali metals (Na, K), etc. Although polyatomic species could in principle be eliminated in RF-only gas-filled reaction cells (e.g. U.S. Pat. Nos. 5,767,512, 7,230,232), such reactions are highly analyte dependent, could affect metals of interest and generally result in losses of these ions of interest. This is especially noticeable for imaging applications where the starting amount of analyte is limited from the start.
ICP MS with laser ablation (LA/ICPMS) is known to have a negligible contribution of polyatomic species and therefore became one of preferred methods for elemental imaging of tissues as shown for example in WO2010/133196, DE10354787, WO0151907, WO02054057, U.S. Pat. No. 8,274,735 WO 2014/063246, WO2015128490 and others. An acquisition rate of up to several tens of pixels/second with micrometer (μm) spatial resolution has been demonstrated. Even with such rate, several hours are still needed for the acquisition of a single image. Further increases in acquisition speed, however, are limited by the temporal spreading of the signal due to spreading of the sample plume during its transport from the surface to the ICP torch, as most of the transport process takes place at atmospheric pressure and at low transport velocities. Atmospheric pressure is essential for ICP operation. Along with this spreading, transfer lines may get coated with aerosol formed by sample material, thus resulting in carryover and contamination of the sample introduction unit. With higher throughput required by any clinical application, excessive contamination will drive the costs of analysis and service time.
Transfer of the ionisation process into vacuum as known in the art for SIMS or laser plasma ionisation approaches results in a very long scanning process due to a relatively low current of generated ions of interest and hence long exposure times being required.
Such low current of generated ions is often caused not so much by ionising agent or low efficiency of secondary ion generation but rather by the relatively low concentration of natural elements or tags in the cell/tissue matrix. This also precludes utilising other methods of multi-channel elemental imaging such as SES, micro X-ray fluorescence (μXRF), etc. Another problem is the rapid contamination of the vacuum chamber and analyzer components with the organic matrix material. For example, analysis of just one typical 5 μm-thick tissue section of 100 mm2 area could completely contaminate an instrument if fully utilised for analysis in order to satisfy sensitivity requirements. In the case of SIMS, there is an added problem of relatively slow rate of sample removal that decelerates analysis of typical tissue samples which are often at least 3-5 micrometers thick.
In the field of isotope ratio mass spectrometry (IRMS), especially where the isotope ratio analyzer is interfaced to a gas chromatography (GC) or liquid chromatography (LC) separation stage, samples are oxidised to produce gases such as CO2, NOx, H2O, which are analysed to determine isotope ratios of elements such as C, N and/or O. The oxidation may take place in a combustion oven (e.g. in GC-IRMS), as described in Z. Muccio and G. P. Jackson, Isotope ratio mass spectrometry, Analyst 134 (2009) 213-222, or it may involve a wet chemical oxidation process (e.g. in LC-IRMS), as described in C. Osburn and G. St-Jean, Limnology and Oceanography: Methods 5 (2007) 296-308. “Dry” oxidation e.g. by UV-ozone, is also routinely used for removal of contaminations on surfaces of semiconductors, glass, etc.
The present invention has been made against this background.